Factory automation and Industrial Process Control require monitoring and maintaining process variables at the appropriate levels in order to make decisions and drive actions. A sensor may be used to measure a process parameter such as temperature, pressure, etc. A transmitter receives a signal from the sensor and transmits a value indicative of the sensor's reading to a programmable logic controller (PLC).
The PLC may be a substantial distance from the sensor and its transmitter. Many installations use a “current loop” to communicate a value from sensor to PLC. In a current loop-based system, the current loop is a two-wire interface which is powered by a voltage source at or near the PLC but whose current is controlled by the transmitter in relation to the sensor value to be transmitted. The PLC determines the magnitude of the current on the loop to recover the transmitted sensor value.